For example, Japanese Unexamined Patent Publication No. H11-22515 discloses an electronic engine control system of a vehicle, which achieves a relatively good engine response upon driver's operation of an accelerator and thereby implements relatively good drivability of the vehicle. In this engine control system, a demanded torque (a target torque), which is demanded by the driver, is computed based on an accelerator opening degree and an engine rotational speed. Then, a target throttle opening degree (a target cylinder air charge quantity) of a throttle valve is computed based on the target torque, and an actual throttle opening degree of the throttle vlave is controlled to the target throttle opening degree. This control system includes a phase lead compensator, which compensates a charging delay of intake air, which has passed through the throttle valve.
Furthermore, Japanese Unexamined Patent Publication No. 2002-309990 recites another system, which includes a feedback control system (a closed loop system) that compensates a target throttle opening degree through a feedback operation based on a difference between a target cylinder air charge quantity and an estimative actual cylinder air charge quantity.
As described above, the control system, which controls the throttle opening degree (the cylinder air charge quantity), includes the phase lead compensator that compensates the charging delay of the intake air, which has passed the throttle valve. Thus, when a noise is added to an input of the control system, the system becomes unstable due to the phase lead compensator.
To address this disadvantage, a noise filter is applied to the input of the control system. Through use of this filter, in the steady operational period of the engine, the phase lead compensation becomes zero (or the phase lead compensation is forcefully changed to zero in the steady operational period of the engine), and thereby the system is stabilized. However, the recent study of the inventors of the present invention reveals that the control state becomes unstable in a moderate transient operational period of the engine due to influences of the following fluctuations (1) and (2) to cause hunting of the target throttle opening degree.
(1) Fluctuations of the target cylinder air charge quantity per unit time.
(2) Fluctuations of a computed value (a map value) of a charging efficiency η, which is caused by fluctuations of, for example, the engine rotational speed and the valve timing.
In this case, the fluctuations during the computation will not cause a substantial problem. However, when the target throttle opening degree shows the hunting, the motor of the electronic throttle system will be operated according to the hunting target value. Thus, the unnecessary operation is performed. This unnecessary operation may result in, for example, the deteriorated fuel consumption, the deteriorated durability of the electronic throttle system and the deteriorated drivability of the vehicle.
As described above, the control system, which includes the phase lead compensator, is sensitive to the noise applied to the input of the control system. Thus, the noise filter is applied to the input of the control system to achieve the stability of the control system. However, the input of the control system includes not only the target cylinder air charge quantity but also other operational parameters (e.g., the engine rotational speed, the valve timing), which may have negative influences on the charging efficiency η. Thus, when the filter needs to be applied to each of the hunting factors, the multiple filters need to be applied. This results in the deterioration in the response. That is, the filter acts as a phase lag compensator. Thus, when the number of the filters is increased further, the response is delayed further.